1. Field of the Invention
The present invention relates to semiconductor integrated circuit devices, particularly those which include a polarity detection circuit which detects the polarities of direct current voltages on a plurality of transmission lines each transfer both a signal and the direct current voltage.
2. Description of the Prior Art
FIG. 6 is a block diagram showing transmission lines, such as a home bus, and apparatuses connected thereto. In FIG. 6, indicated at reference characters 50a to 50d are apparatuses which perform various functions when applied with a dc voltage. Indicated at 51 and 52 are transmission lines connected to the apparatuses 50a to 50d to supply a dc voltage to the apparatuses 50a to 50d while superimposing signals which are routed between the apparatuses 50a to 50d on the dc voltage and transmitting the superimposed signals. Indicated at 53 is a dc power source which is connected to the transmission lines 51 and 52 through resistors 54 and 55 to supply a dc voltage to the apparatuses 50a to 50d. Indicated at 56 to 58 are a resistor and capacitors for suppressing distortion in the waveforms of signals on the transmission lines 51 and 52. As shown in FIG. 6, signals from the apparatuses 50a to 50d and dc voltages are applied between the transmission lines 51 and 52. Depending on the circumstances, the dc power source 53 may be connected to the transmission lines 51 and 52 in reverse. The polarities of the applied dc voltages, however, in most cases cannot be known by the appearances of the transmission lines 51 and 52. For instance, the transmission line 51 is not necessarily at higher level than the level of the transmission line 52, but rather, the transmission line 51 receives no dc voltage from the dc power source 53 in some cases. For this reason, the apparatuses 50a to 50d each comprise a circuit for detection of the polarities of the dc voltages applied to the transmission lines 51 and 52, as is common in the art.
FIG. 7 is an expanded view showing the part of FIG. 6 which is enclosed in the dashed line. The apparatus 50a comprises power source terminals 65 for receiving a dc voltage and a signal input/output terminal 66 for transmission of a signal through the capacitors 63 and 64. The signal input/output terminal 66 is connected, for example, to a driver/receiver circuit 61 to enable data transfer. To the power source terminals 65, a polarity detection circuit 60 is connected which receives dc voltages via the power source terminals 65 and detects the polarities of the dc voltages so that use of a dc voltage in the apparatus 50a is made possible. If power source is to be provided to the apparatus 50a through the transmission lines 51 and 52, a power source circuit is necessary which is connected to the apparatus 50a through a bridge circuit. In FIG. 7, such a power source circuit is not shown.
FIG. 8 is a circuitry diagram showing an example of the polarity detection circuit of FIG. 7. In FIG. 8, the transmission lines are labeled 51 and 52 and the polarity detection circuit is labeled 60. Indicated at reference numeral 75 is a current limit resistor connected at one end to the transmission line 52 to limit a current flowing in the polarity detection circuit 60. Indicated at 72 and 73 are protection diodes which are connected between the other end of the current limit resistor 75 and the transmission line 51. At reference numeral 69, a diode is shown which has an anode connected to the other end of the current limit resistor 75. Indicated at 67 is a photo coupler having an anode of its light emitting diode connected to a cathode of the diode 69, a cathode of its light emitting diode connected to the transmission line 51 and an emitter of its photo transistor grounded. A pull-up resistor is labelled 74a which has one end connected to a collector of the photo transistor of the photo coupler 67 and the other end connected to the power source. When a voltage on the transmission line 51 is lower than a voltage on the transmission line 52 and larger in absolute value than the sum of a threshold voltage of the diode 69 and a threshold voltage of the photo coupler 67, a current flow is initiated which flows from the transmission line 52 to the transmission line 51 through the diode 69, the photo coupler 67 and the current limit resistor 75. If this happens, an output signal POI from a polarity detection signal output terminal 76 changes from "H" level to "L" level.
Indicated at reference numeral 70 is a diode which has a cathode connected to the other end of the current limit resistor 75. Indicated at 68 is a photo coupler having a cathode of its light emitting diode connected to an anode of the diode 70, an anode of its light emitting diode connected to the transmission line 51 and an emitter of its photo transistor grounded. A pull-up resistor is labelled 74b which has one end connected to a collector of the photo transistor of the photo coupler 68 and the other end connected to the power source. When a voltage on the transmission line 51 is higher than that on the transmission line 52 and larger in absolute value than the sum of a threshold voltage of the diode 70 and a threshold voltage of the photo coupler 68, a current start flowing from the transmission line 51 to the transmission line 52 through the diode 70, the photo coupler 68 and the current limit resistor 75. As a result, an output signal PO2 from a polarity detection signal output terminal 77 changes from "H" level to "L" level. The polarities of voltages on the transmission lines 51 and 52 are judged from the output signals PO1 and PO2 which are available from the polarity detection circuit 60. The polarity detection circuit 60 is insulated from the circuits beyond the transmission lines 51 and 52 and the output terminals 76 and 77 by the existence of the photo couplers 67 and 68.
Having such a construction as described above, the conventional semiconductor integrated circuit device requires discrete parts such as photo couplers to be used therein. Hence, an increased number of parts are necessary, resulting in a larger device size and a high product price.